The present invention relates to an ink jet recording medium for pigment ink, which is excellent in color reproducibility such as developed color density, contrast and color tone and has an excellent pigment fixing property, and a recording method therefor.
Along with rapid dissemination of digital cameras, or computers, hard copy technologies to record prints or images output therefrom on paper sheets, etc., have been rapidly developed. Thus, it has now been possible to obtain quality and performance close to the objective levels of silver salt photographs with respect to color reproducibility, image density, gloss, weather resistance, etc. As such recording systems for hard copies, an ink jet system, a sublimation type thermal transfer system and an electrostatic transfer system may, for example, be mentioned as typical examples.
Among them, the ink jet system has merits in that the apparatus is relatively small in size, and the running cost is low, and together with the sublimation type thermal transfer system or the like, it is considered to be the main system for hard copies. The ink jet system is a system wherein ink droplets comprising a colorant and a large amount of a medium, are jetted at a high speed from a nozzle to a recording medium. Printers of such ink jet system have become widely used in recent years, since full color and high speed are thereby easy, and printing noises are thereby low.
As a colorant for ink in the ink jet recording system, it has been common in most cases to employ a water-soluble dye excellent in color reproducibility (Gamut). However, such a dye ink has had a difficulty in fastness including light resistance and water resistance. Therefore, recently, it has been proposed to use a pigment ink wherein the colorant is a pigment, which is already been widely used. With the pigment ink, the above-mentioned difficulty in light resistance, water resistance, etc. which used to be a problem with a conventional dye ink, can be solved, and ink running as observed when a dye ink is used for printing on a recording sheet, can be remarkably minimized.
Further, the pigment ink is basically the same coloring material as the ink used in plate printing such as offset printing or gravure printing, and since the hue of the developed color is similar, it can be used also as a so-called color proof in test printing for color correction in such plate printing.
However, when a pigment ink is used for printing on a recording medium such as paper which is used for a conventional dye ink, there has been a problem that the color reproducibility such as the developed color density, contrast or color tone, tends to be inadequate, or the ink tends to be easily removed when the printed portion or the image portion is rubbed after being printed. Further, in contrast with a dye ink wherein a dye is dissolved in water, a pigment ink is one wherein fine particulate water-insoluble pigment particles are dispersed in a medium such as water and thus has a problem that as the medium evaporates, abnormal jetting is likely to occur due to an increase of the viscosity of the ink in the vicinity of the ink jet nozzle, and in the worst case, clogging of the nozzle is likely to result.
In recent years, there have been proposals for means to solve such difficulties in ink jet printing employing such a pigment ink, for example, in JP-A-11-78225, JP-A-10-119422, JP-A-10-166717 and JP-A-9-123593. Among them, JP-A-11-78225 proposes an ink jet recording medium wherein in order to improve fixing of a pigment ink, on an ink-receiving layer formed on a paper substrate, a pigment fixing layer having a certain specific physical property is further formed. However, due to the multilayer structure, the production tends to be correspondingly cumbersome, and the cost increases accordingly.
It is an object of the present invention to provide an ink jet recording medium for pigment ink having an ink-receiving layer on the surface of a substrate, which is excellent in the color reproducibility such as the developed color density, contrast and color tone and which has an adequate pigment ink fixing property in the ink-receiving layer without necessity of forming a special pigment fixing layer, while maintaining merits of using a pigment ink, such as light resistance, water resistance and little running, and an ink jet recording method employing such a recording medium.
The present inventors have conducted various researches to accomplish the above object and as a result, have found that as between ink jet printing employing a pigment ink and ink jet printing employing a conventional dye ink, the natures and characteristics of the respective inks are different, and accordingly, totally different characteristics are required for recording media used for the respective printing methods. Namely, FIGS. 1 and 2 are enlarged cross-sectional views of recording media, which schematically illustrate the respective behaviors of colorants (S) in the case of a dye ink and in the case of a pigment ink, when ink jet printing was carried out by using recording media having different surface conditions of the ink-receiving layers.
When ink jet printing is carried out by using three types of recording media i.e. a recording medium (A) having an ink receiving layer having a high surface roughness, a recording medium (B) having an ink receiving layer having an intermediate surface roughness and a recording medium (C) having an ink receiving layer having a smooth surface, in the ink jet printing employing a conventional dye ink as shown in FIG. 1, the colorant (S) made of a dye is dissolved in a medium such as water constituting the ink and therefore will penetrate together with the ink medium into the interior of the recording medium immediately upon printing and will settle on the surface or in the surface layer of the ink receiving layer (I) of the recording medium.
As a result, the quality of prints or images formed by the ink jet printing by a dye ink is such that when the surface of the ink receiving layer is very rough as in the recording medium (A), the developed color density is low, and the gloss is low, and in the case of the intermediate surface roughness as the recording medium (B), the developed color density is moderate, and the gloss is also moderate. Whereas, in the case of an ink receiving layer having a high surface smoothness as in the recording medium (C), excellent prints and images can be obtained where the developed color density is high and the gloss is also high.
On the other hand, in the case of ink jet printing by a pigment ink, the colorant (S) made of a pigment is in a dispersed state as fine particles without being dissolved in an ink medium, and when the pigment ink is printed, while the ink medium will penetrate into the interior of the recording medium, the colorant will not penetrate into the interior of the recording medium and will deposit on the surface or in the vicinity of the surface of the ink receiving layer (I) of the recording medium.
In such a case, when the ink receiving layer has a high surface smoothness as in the recording medium (C), there is no so-called anchor effect, and pigment particles of the colorant merely deposit on the surface of the recording medium, whereby no adequate fixing property can be obtained, and they are likely to fall off even by slight abrasion. Further, the resulting images tend to have a metallic gloss, thus leading to a so-called bronzing phenomenon. On the other hand, in a case where the ink receiving layer has a rough surface as in the recording medium (A), the pigment particles as the colorant will be embedded in the roughened surface of the ink receiving layer, whereby only part of pigment ink particles will be exposed on the surface, whereby the developed color density tends to be low. Thus, in the case of ink jet printing by a pigment ink, when the ink receiving layer has an intermediate suitable surface roughness as in the recording medium (B), the pigment particles will be properly fixed on the surface, whereby the developed color density will be sufficient and a high quality where they will not be removed even by abrasion, can be obtained.
The present invention has been made as a result of combining the above findings with the surface characteristics of the ink receiving layer of a recording medium, and has the following construction.
(1) An ink jet recording medium for pigment ink, which comprises a substrate and a porous ink receiving layer for pigment ink, formed on the surface of the substrate, wherein the ink receiving layer comprises a pigment and a binder and has an average surface roughness (Ra) according to JIS B0601 of from 0.2 to 2.0 xcexcm
(2) An ink jet recording method employing a pigment ink, which comprises ink jet printing a pigment ink to an ink jet recording medium which comprises a substrate and a porous ink receiving layer for pigment ink, formed on the surface of the substrate, wherein the ink receiving layer comprises a pigment and a binder and has an average surface roughness (Ra) according to JIS B0601 of from 0.2 to 2.0 xcexcm.
On the other hand, the present inventors have found that as the surface characteristics of the ink receiving layer, when the specular gloss at 20xc2x0 and the distinctness of image gloss of the surface are within certain specific ranges, pigment particles will be properly fixed on the surface, whereby the developed color density will be adequate, and a high quality where pigment particles will not be removed even by abrasion, will be obtained. This is considered to be explained in such a way that when the surface of the ink receiving layer has the above-described characteristics, in the ink jet printing by a pigment ink, the ink receiving layer as in the above-mentioned recording medium (B) has a proper surface roughness of an intermediate level.
Thus, the present invention is made based on the above discovery and has the following construction.
(3) An ink jet recording medium for pigment ink, which comprises a substrate and a porous ink receiving layer for pigment ink, formed on the surface of the substrate, wherein the ink receiving layer comprises a pigment and a binder, and the surface of the ink receiving layer has a specular gloss at 20xc2x0 of from 2 to 18% and a distinctness of image gloss of at most 20.
(4) An ink jet recording method employing a pigment ink, which comprises ink jet printing a pigment ink to an ink jet recording medium which comprises a substrate and a porous ink receiving layer for pigment ink, formed on the surface of the substrate, wherein the ink receiving layer comprises a pigment and a binder, and the surface of the ink receiving layer has a specular gloss at 20xc2x0 of from 2 to 18% and a distinctness of image gloss of at most 20.